Heretofore, thermistors comprising oxides of Mn and Co as their main components have been widely used. They include compositions of Mn-Co system oxide, Mn-Co-Cu system oxide, Mn-Co-Ni system oxide and Mn-Co-Ni-Cu system oxide, which have been used as general purpose disc shape thermistors for such applications as in temperature compensation, etc. These thermistors give, as a characteristic of such materials, specific resistances from ten and several .OMEGA.-cm to one hundred and several tens k.OMEGA.-cm for use mainly in a temperature range from -40.degree. C. to 150.degree. C. However, demand for their use as temperature sensors has recently grown larger; thus, thermistor sensors which are usable at higher temperatures have been in demand.
As a first step, a demand has been raised for thermistor sensors which are usable at temperatures up to 300.degree. C. for temperature control of petroleum combustion equipment. In order to deal with this situation, materials with high specific resistances have been used as materials of thermistors in the place of conventional materials comprising oxides of Co-Mn as their main components and until now Mn-Ni-Al system oxide semiconductors (Japanese Patent Gazette Patent Laid-Open No. Sho 57-95603) and Mn-Ni-Cr-Zr system oxide semiconductors (Specification of U.S. Pat. No. 4,324,702) offered by the present inventors have been put into practical use.
With regard to the construction of the sensor, sloughing conventional structure of the disc shape thermistor molded of resin, the object of shielding it from high temperature atmosphere has been attained by sealing a thermistor element of such a very minute size as 500 .mu.m.times.500 .mu.m.times.300 .mu.m (t) in a glass tube or by coating glass on the thermistor element by way of dipping. On the other hand, just as the disc shape thermistors, bead shape thermistors have been improved in heat resistance by glass-coating.
However, a demand for thermistor sensors which are usable at still higher temperatures has not been abated, there is a strong demand for sensors at such temperatures as above 300.degree. C., 500.degree. C. or up to 700.degree. C. These demands can not be met with the conventional materials because of the following two problems involved: (1) their specific resistances, one of characteristics of thermistor materials, are low; that is, resistances required for operation of equipment at intended temperatures can not be obtained, and (2) they are not reliable because their resistance changes with time at high temperatures and thus exceeds the required 5% (500.degree. C., 1000 Hr).
On the other hand, materials used at such high temperatures as 700.degree. C.-1000.degree. C., stabilized zirconia (ZrO.sub.2 -Y.sub.2 O.sub.3, ZrO.sub.2 -CaO, etc.), Mg-Al-Cr-Fe system oxide compositions, etc., have been developed. However, as these oxide materials require such high sintering temperatures above 1600.degree. C.; they could not be sintered, using ordinary electric furnaces (operatable at 1600.degree. C. max.). Moreover, even sintered materials give large resistance changes with time at high temperatures, being as large as 10% (1000 Hr) as reported for even the very stable ones, and therefore, improvement in reliability is further sought.
To solve this problem, new materials have already been developed in Japan, but they are still in the evaluation stage (Mn-Zr-Ni system oxide: Japanese Patent Gazette, Patent Laid-Open No. Sho 55-88305 (Ni.sub.x Mg.sub.y Zn.sub.z) Mn.sub.2 O.sub.4 -spinel type: ibid. Patent Laid-Open No. Sho 57-88701 (Ni.sub.p Co.sub.q Fe.sub.r Al.sub.s Mn.sub.t)O.sub.4 -spinel type: ibid. Patent Laid-Open No. Sho 57-88702).